Field of the Invention
The present invention relates to wireless communication, and more particularly, to a method for transmitting a reference signal in a wireless communication system.
Discussion of the Related Art
In next generation multimedia mobile communication systems, which have been actively studied in recent years, there is a demand for a system capable of processing and transmitting a variety of information (e.g., video and packet data) in addition to the early-stage voice service. In order to maximize efficiency of a limited radio resource in a wireless communication system, methods for more effectively transmitting data in spatial and frequency domains have been provided.
Orthogonal frequency division multiplexing (OFDM) uses a plurality of orthogonal subcarriers. Further, the OFDM uses an orthogonality between inverse fast Fourier transform (IFFT) and fast Fourier transform (FIT). A transmitter transmits data by performing IFFT. A receiver restores original data by performing FFT on a received signal. The transmitter uses IFFT to combine the plurality of subcarriers, and the receiver uses FFT to split the plurality of subcarriers. According to the OFDM, complexity of the receiver can be reduced in frequency selective fading environment of a broadband channel, and spectral efficiency can be increased when selective scheduling is performed in frequency domain by using channel characteristic which is different from one subcarrier to another. Orthogonal frequency division multiple access (OFDMA) is an OFDM-based multiple access scheme. According to the OFDMA, efficiency of radio resources can be increased by allocating different subcarriers to multi-users.
To maximize efficiency in spatial domain, the OFDM/OFDMA-based system uses multi-antenna technique which is used as a suitable technique for high-speed multimedia data transmission by using a plurality of time and frequency resources in the spatial domain. The OFDM/OFDMA-based system also uses channel coding scheme for effective use of resources in time domain, scheduling scheme which uses channel selective characteristic among a plurality of users, hybrid automatic repeat request (HARQ) scheme suitable for packet data transmission, etc.
Channel estimation needs to be reliable to ensure high-speed data transmission. It is important to design a reference signal, which is used for channel estimation, in order to increase the reliability of channel estimation. The reference signal is the signal known to both the transmitter and the receiver, and is also referred to as a pilot. A channel condition may vary depending on time and frequency. Therefore, the reference signal needs to be designed to cope with the channel condition flexibly, thereby increasing the reliability of channel estimation.
In general, the reference signal uses a fixed spreading code in time-frequency domain. Orthogonality of the spreading code is used to distinguish users. Since the reference signal is transmitted by using fixed radio resources, orthogonality between reference signals may be impaired when the channel condition changes rapidly. This may cause intra-cell interference or inter-cell interference. In this case, channel estimation may be inaccurate. If the same reference signal structure is used in a situation where channel characteristics of users are different from one another, it is difficult to increase the reliability of channel estimation. In addition, the reference signal has to be allocated in a flexible manner in order to increase a system capacity because the number of orthogonal codes is limited when a radio resource is limited and also because user accommodation capability is determined according to the number of available orthogonal codes.
Therefore, there is a need for a method for designing a reference signal in a flexible manner, whereby the user accommodation capability can be increased when radio resources are limited and whereby channel estimation and inter-cell interference can be effectively dealt with.